In a statement distributed just before the announcement at the Red Hook Community Justice Center, de Blasio called Bratton "a proven crime-fighter."

Bratton, in his own statement, said, "Mayor-elect de Blasio's priorities are my priorities."

Here's what to expect from the once and future commissioner.

A (Relatively) Easy Transition

Expect Bratton to get right to work in a department he already knows his way around. He served as commissioner from 1994 to 1996. Before he was appointed to the position by Rudy Giuliani, he served as chief of the New York City Transit Police too. Bratton, who is 66, also did stints as chief of police of Los Angeles (2002-2009) and superintendent of Boston police ('86-'90).

During his relatively short, 27-month stint as the NYPD's top cop, Bratton took credit for a dramatic drop in crime attributed to the "Broken Windows" theory of policing he instituted, a kind of trickle-down theory of crime that held prosecuting minor crimes would stave off major ones.

Bratton also introduced CompStat to the NYPD. The program, which uses statistical models to determine where crime will occur, has since been replicated in departments around the country.

Back in the mid '90s, Bratton's growing profile -- he signed a $350,000 book deal, and received a higher approval rating than any other politician in the city (the mayor included) -- ultimately put him at odds with Giuliani and resulted in his ouster.

Recently though, Bratton's old boss had only nice things to say about his former commissioner. Giuliani called him an "exceptional law enforcement leader" at a recent event.

Conflicts of Interest

Bratton's sparkling record hasn't just made him popular among politicians like de Blasio, Giuliani, and Los Angeles' Herb Wesson (who proposed a city charter amendment to allow Bratton to serve a third term). It's made him popular with private corporations too.

Since leaving the LAPD in 2009, Bratton has worked with several companies that sell -- or are interested in selling -- gear to the NYPD, including Manhattan-based Kroll Inc. Kroll holds a $1.7 million dollar contract with the NYPD, and Bratton remains a "senior adviser" there. Bratton also serves on the board of ShotSpotter, the makers of gunshot-detection systems, a company that has paid firms to lobby the NYPD on its behalf.

Expect Bratton to make tackling traffic fatalities, which are close to on par with murders in New York City this year, a priority for the department. At an event in November, Bratton pledged to apply the same determination to traffic fatalities that he used to tackle crime in the 90s. "More can be done in this critical area. The time for this issue has come," Bratton said.

What not to expect: an End to the Stop-and-Frisk Era

One thing no one should expect from Bratton? A total abolishment of the stop-and-frisk policies that de Blasio campaigned so forcefully against. Speaking to the Wall Street Journal in January, Bratton called stop-and-frisk "the most basic, fundamental and necessary tool of American policing. We cannot function effectively without it."